Beam focusing magnet



May 11, 1965 R. c. HERGENROTHER 3,183,398

BEAM FOCUSING MAGNET Filed Aug. 4, 1960 2 Sheets-Sheet 1 I NVEN TOR RUDOLF C. HEHGENROTHEH ATTORNEY M y 11, 1965 R. HERGENROTHE-R 3,183,398

BEAM FOCUSING MAGNET Filed Aug. 4, 1960 2 Sheets-Sheet 2 INVENTOH RUDOLF G. HEHGENROTHER ATTORNEY United States Patent "ice 3,183,398 BEAM FOCUSING MAGNET Rudolf C. Hergenrother, West Newton, Mass, assignor to Raytheon Company, Lexington, Mass, at corporation of Delaware Filed Aug. 4, 1960, Ser. No. 47,499 14 Claims. (Cl. 315-35) This invention relates to beam focusing magnets and more particularly to methods and means of construction of a structure producing magnetic fields periodicallyspaced along a beam of charged particles serving to focus the beam.

In the past, periodic magnetic focusing fields have been employed for focusing a beam of charged particles moving Within a tube. The charged particles pass through periodically-disposed magnetic fields which may be produced by, for example, numerous small cylindrical magnets arranged in a line. While the magnitude of each periodic field is higher than that required of a non-periodic magnetic focusing'field, there are, nevertheless, certain advantages to be gained by employing periodic fields. For example, a substantial reduction in magnet weight is achieved and the numerous small magnets employed may be made an integral part of the structure of the tube which encloses the beam.

Two types of periodic magnetic fields have been employed to focus a beam of charged particles. These are sometimes called the axially symmetric periodic magnetic fields and the quadrupolar periodic magnetic fields. Both types of fields and means for producing them are discussed in pages 436 to 444 of the April 1954 (volume 25), edition of the Journal of Applied Physics. The present invention relates to axially symmetric periodic magnetic fields and includes novel features not employed in the past.

The present invention includes a structure for producing axially symmetrical periodic magnetic fields and a method of making the structure. Each of the periodic fields has substantial components parallel to the direction of motion of the charged particles moving axially to the structure and such components of adjacent periodic fields are oppositely directed. One structural embodiment of the present invention shown herein includes a cylinder of ferromagnetic material, such as Alnico, magnetized circumfereutially as a quadrupole with magnetically permeable members radially arranged inside for coupling magnet lines from the ferromagnetic cylinder to a beam or" charged particles moving axially within the cylinder. In other embodiments of the present invention, the magnetically permeable members are all bent or flexed in the same direction to reduce the effects of thermal expansion and contraction of the members.

One method for making the structure includes, but is not limited to, the steps of aligning microscopic magnetic dipoles in the Alnico cylinder by heating the cylinder above the Curie temperature, conducting a strong DC. current axially to the cylinder and slowly lowering the temperature. By this process the magnetic dipoles in the Alnico cylinder will all be aligned either clockwise or counterclockwise perpendicular to the cylinder axis and tangential to the walls.

Next, magnetically permeable members of, for example, soft iron are arranged inside the ferromagnetic cylinder along the cylinder axis with their lengths disposed radially thereto and adjacent members preferably disposed perpendicular to each other. Finally strong D.C. currents are conducted in different directions down the length of the Alnico cylinder as a quadrupole so that the magnetically permeable members conduct magnetic lines in radial directions toward or away from the axis of the Alnico cylinder.

Other features of the present invention will be apparent 3,3835% Patented May 11, 1965 from the following specific description of embodiment taken in conjunction with the figures in which:

FIG. 1 illustrates an arrangement of the present invention to form a portion'of the walls of a traveling wave tube for producing periodic magnetic fields within the tube;

FlG. 2 illustrates a cylindrical magnet electrically magnetized as a quadrupole having magnetized members contacting the inner walls thereof;

FIG. 3 illustrates some of the steps whereby dipoles in the ferromagnetic cylinder are orientated before assembly and magnetization;

FIG. 4 illustrates an end view of the device in FIG. 2 showing the direction of magnetization caused by D.C. magnetizing currents within the cylinder; and

FIG. 5 illustrates an embodiment wherein arms of the magnetized members fixed axially within the cylinder are bowed in a given direction to substantially eliminate changes in the magnetic fields due to expansion and contraction of the arms.

Turning first to FIG. 1 there is shown a traveling Wave tube 1 including a cathode 2 emitting electrons forming a beam 3 and a collector 4 disposed at the other end of the tube for collecting electrons. The beam 3 formed by the electrons emitted from cathode 1 extends the length of slow wave propagating structure 5 which is represented as a helix. At the cathode end of the tube, the slow wave structure 5 is coupled to input 6a and at the collector end it is coupled to load 6b. While slow wave structure 5 is represented as a helix it is understood. that any of a number of slow wave structures such as the interdigital line, may be substituted therefor.

The walls of tube 1 are formed in part by magnetically permeable cylindrical sections '7 through 11 which are marked N or S indicating that the cylinders are North or South Poles. The cylinders 7 through 11 are spaced from each other by spacers 12 through .15 which are also cylindrical in shape but are preferably composed of nonmagnetic material. Since the magnetically permeable cylinders 7 through 1 1 are alternately North and South, magnetic fields are produced within tube 1, as represented by the broken lines, in directions represented by arrowheads. It is further apparent that these magnetic fields are periodically disposed along the length of the tube and are substantially parallel to the beam along the axis of the tube. Furthermore, while adjacent fields. are oppositely directed, their components along the axis of the tube are parallel to the tube axis and serve to focus the beam of electrons 3 moving along the axis.

The magnetic lines running from adjacent cylindrical sections are conducted through closed magnetically permeable loops formed by the adjacent sections, radial arms, such as 16 and 17 extending therefrom, and ferromagnetic cylinder 18 which is preferably magnetized as a quadrupole. Arms similar to 16 and 17 and not shown in FIG. 1, extend perpendicular to arms 15 and 17 from cylindrical sections 8 and 16.

FIG. 2 illustrates a three-quarter view of the magnetized parts of the structure shown in FIG. 1. This consists of a ferromagnetic cylinder 18, which is preferably made of Alnico, and a plurality of smaller magnetically permeable cylindrical sections such as '7 and 8 each with radial arms, such as 16 and 17, extending therefrom and attached to the inner walls of cylinder 18. The arms, such as 16 and 17, are preferably disposed along the axis of cylinder 13 so that arms from adjacent cylindrical sections are perpendicular to each other as shown in the figure. Substantially near the center of each of the cylindrical sections is an opening such as opening 19 in section 7. These openings are substantially concentric with cylinder 18 forming a space through which a beam of 3 a charged particles may be projected and focused periodically.

Alnico cylinder 18 is magnetized as a quadrupole so that the direction of magnetization therein is perpendicular to the axis of cylinder 18 and to a large degree circumferential. However, the direction of magnetization changes about the circumference in four circumferential positions substantially 90 degrees apart. It is at these points, where the direction of magnetization changes, that arms such as 16 and 17 are located.

The arms are disposed radially to cylinder 18 with adjacent pairs of arms conducting magnetic fields in opposite radial directions. Consequently, the magnetic lines fiow in closed loops such as represented by broken lines 21 and 22 which are directed as indicated by the arrow heads. Magnetic lines which cross the gap between adjacent cylindrical sections, such as 7 and 8, are all directed in the same axial direction and form the periodic magnetic fields suitable for focusing charged particles moving along the axis.

One method for permanently magnetizing ferromagnetic cylinder 11-8 to obtain the periodic magnetic fields between adjacent cylindrical sections 7 and 8 consists of the steps including but not limited to the following:

Heat ferromagnetic cylinder to a temperature exceeding the Curie temperature, then conduct a strong D.C. current along the axis of the cylinder as shown in FIG. 3. Maintain the current while the cylinder cools so that the magnetic field produced by the current, represented by the broken lines 23 in FIG. 3, orientate magnetic dipoles I in the cylinder in the same direction. Next, assemble each of the sections such as 7 and 8 with arms radially disposed within cylinder 18 as shown in FIG. 2. The arms of adjacent sections are preferably disposed perpendicular to each other, as shown. Insert an electrical conductor such as conductor 25 shown in FIG. 2, in each of the four longitudinal passages formed within cylinder 18 by the inner walls of cylinder 18 and the radially extending arms. Apply a large D.C. current to conductor 25 from an impulse source. Conductor 25 carries an electrical current longitudinal t-o cylinder 18 through each of the four passages formed by cylinder 18 and the perpendicularly arranged arms. In the embodiment shown in FIG. 2, the conductor 25 enters one passage at one end and emerges at the other end of cylinder '18 and reenters another of the four passages emerging again at the first end and reentering still a third passage. Upon emerging from the third passage the conductor loops around and enters the :fourth passage, emerging from the fourth passage as shown in FIG. 2. The four parallel sections .of conductor 25 are denoted 25a, 25b, 25c and 25d, as shown. The ends 25a and 25b of conductor 25 are coupled to dilferent potentials so that current flows through the conductor in the direction of the solid line arrows producing magnetic fields about sections 25a and 25d as represented by broken lines 22 and 21, respectively. With current fiowing in the direction of the arrows, a magnetic field is produced which couples from cylinder 18 to arm 16, across the gap from section 7 to section 8 and back to cylinder 18 forming a closed loop. The magnetic field circling section 25a of conductor 25 is represented by a broken line 22 directed as shown by the arrowhead. Magnetic lines 22 and 21 are in the same direction across the gap between sections 7 and 8 and, as a result, section 7 appears as a North Pole and section 8 appears as a South Pole with magnetic lines bridging the gaps between these two pole pieces having substantial components along the axis of cylinder 18. Successive members similar to section 7 with arms 16 and 17, are disposed along the axis of cylinder 18 with their openings concentric to cylinder 18. The successive sections are alternately North and South Poles and so magnetic fields will be created in the gap between each section and adjacent fields will he oppositely directed with substantial components parallel to the axis of cylinder 18 and suitable for focusing an electron beam directed along said axis.

The magnetic fields around each of the sections of conductor 25 in FIG. 2 are shown more clearly by an end view of the structure represented by FIG. 4. The four sections of conductor 25 are shown conducting current either into or out of each of the four passages formed between arms 16 and 17 and the Alnico cylinder 18. Each of the sections of conductor 25 are shown as triangular in cross section so as to more suitably conform to the passage in which they are inserted. The magnetic field around each of these sections of conductor 25 is represented by a broken line loop. The direction of all the magnetic fields in arms attached to a given cylindrical section, such as section 7, are all directed radially toward the section or all radially away from the section. Consequently, the magnetic field bridging the gap between adjacent sections is substantially uniform about the circumference of openings such as 19, with equal intensity of lines emanating from opposite sides thereof to a similar opening in the adjacent section.

After the Alnico cylinder 18 has been magnetically oriented by the impulse source the conductors 25a- 25d may be withdrawn and a permanent quadrupolar magnet 18 will be realized. Where desired, a highly magnetically permeable cylinder 18 of soft iron may be employed with the electrical conductors 25a-25d disposed as shown in FIGS. 2 and 4 to maintain the periodic magnetic fields along the tube axis. The conductors are then retained in this position in the completed tube and when energized an electromagnetic equivalent of the permanent quadrupolar magnetic cylinder will result.

FIG. 5 illustrates an embodiment of the basic structure shown in FIGS. 1, 2 and 4 showing, in cross section per manently magnetized Alnico cylinder 18, magnetized arms 16 and 17 extending from cylindrical section 7 Withother sections 8, 26 and 27 disposed along the axis of cylinder 18 in a similar manner. Each of the sections 7, 8, 26 and 27 are separated by non-magnetic rings such as 28 and 29 so that cylindrical sections 7, 8, 26 and 27 and nonmagnetic such as rings 28 and 29 form the walls of a tube concentric with cylinder 18. As shown in FIG. 5 the radial arms such as 16 and 17, which extend toward the inner walls of cylinder 18, are bowed so that the cylindrical sections supported by the arms will be displaced uniformly along the axis as the arms expand and contract thermally. Consequently, the periodic magnetic fields within the tube will not be displaced with respect to each other as a result of such thermal expansion and contraction.

While there is described herein various methods and means of construction of a structure for producing periodic magnetic focusing fields along the path of a beam of charged particles, it is to be understood that these are made only by way of example and other steps could be included in the method described to yield a similar structure producing similar periodic magnetic focusing fields without deviating from the spirit or scope of the invention as set forth in the following claims.

What is claimed is: p

1. A magnetic focusing device comprising a relatively large tubular highly magnetically permeable body, a plurality of relatively small tubular highly magnetically permeable bodies disposed coaxially within said large body, individual magnetically permeable radial support members coupling each of said small bodies to the inner walls of said large body, so that adjacent support members are disposed transverse to each other and means conducting electric current longitudinally Within said first tubular body to induce magnetic fields between adjacent small bodies which magnetize said tubular body as a quadrupolar magnet.

2. A device as in claim 1 including at least four elec trical conductors disposed within said large tubular body 7 space having substantial components along the axis of 2,860,278 said device. 2,876,373 2,956,193 References Cited by the Examiner 2 991 332 UNITED STATES PATENTS 2,504,870 4/50 Oliver 148-103 2,825,670 3/58 Adams et a1 148-403 8 Cook et a1 31384 X Veith et a1 31384 De Wit 313-84 Yasuda 313-84 5 GEQRGE N. WESTBY, Primary Examiner.

RALPH G. NILSON, Examiner.

V and energized so that oppositely disposed conductors conduct current in the same longitudinal direction.

3. A device as in claim 1 including two pairs of conductors each pair conducting electrical current in opposite directions longitudinally within said large body.

4. A device as in claim 1 including a single electrical conductor passing longitudinally through and within said large tubular body at least four times conducting a current longitudinally therein in opposite directions each successive time.

5. A device as in claim 1 including at least two U- shaped electrical conductors each conducting electrical current longitudinally through said first tubular body, first one direction and then in the opposite direction.

6. A device as in claim 1 including a plurality of electrical conductors each disposedlongitudinally within said large tubular body in longitudinal spaces formed between said support members and said'large tubular body.

7. A device as in claim 1 wherein each of said support members is bent so that thermal expansions and contrac tions of said support members will result in equal axial displacement of said plurality of small tubular bodies within said large tubular body.

8. A device as in claim 1 including a plurality of nonmagnetic rings each disposed between and sealed to adjacent of said small tubular magnetically permeable bodies .to form a sealed tubular space disposed coaxially within said large magnetically permeable tubular body.

9. In a traveling wave tube including means projecting a beam of electrons through an interaction space in energy exchanging relationship with waves propagating therein, means for focusing said electrons in said space and enclosing said space comprising a plurality of annular members of high magnetic permeability spaced apart along the axis of said device substantially coaxial therewith, a plurality of annular spacers of low magnetic permeability disposed between and sealed to said members, said members and spacers thereby enclosing said interaction space, a quadrupolar magnet of larger crosswise dimension than said members disposed coaxial with said device enclosing said members and spacers, and a plurality of radial arms of high magnetic permeability extending from each of said members to the inner walls of said quadrupolar magnet orientated to produce periodic magneticfields within said space having substantial components along the axis of said device.

10. In a traveling wave tube including means projecting a beam of electrons through an interaction space in energy exchanging relationship with waves propagating therein, means for focusing said electrons in said space and enclosing said space comprising a plurality of annular members of high magnetic permeability spaced apart along the axis of said device substantially coaxial therewith, a plurality of annular spacers of low magnetic permeability disposed between and sealed to said'members, said members and spacers thereby enclosing said interaction space, a quadrupolar magnet of larger crosswise dimension than said members disposed coaxial with said device enclosing said members and spacers, and two radial arms of high magnetic permeability extending from each of said members to the inner walls of said quadrupolar magnet, said arms from adjacent members extending to poles of opposite sign to produce periodic magnetic fields within said space having substantial components along the axis of said device.

11. In a traveling wave tube including means projecting a beam of electrons through an interaction space in energy exchanging relationship with waves propagating therein, means for focusing said electrons in said space and enclosing said space comprising a plurality of annular members of high magnetic permeability spaced apart along the axis of said device substantially coaxial therewith, a plurality of annular spacers of low magnetic permeability disposed between and sealed to said members, said members and spacers thereby enclosing said interaction space,

6 i a quadrupolar magnet of larger crosswise dimension than said members disposed coaxial with said device enclosing said members and spacers, and two radial arms of high magnetic permeability extending from each of'said members to identical poles of said quadrupolar magnet, said arms from adjacent members extending to poles of opposite sign to produce periodic magneticfields within said space having substantial components along the axis of said device.

12. In a traveling wave tube including means projecting a beam of electrons through an interaction space in energy exchanging relationship with waves propagating therein, means for focusing said electrons in said space and enclosingsaid space comprising a plurality of annular members of high magnetic permeability spaced apart along the axis of said device substantially coaxial therewith, a plurality of annular spacers of low magnetic permeability disposed between and sealed to said members, said members and spacers thereby enclosing said interaction space, a tubular member of larger crosswise dimension than said members disposed coaxial with said device enclosing said members and spacers, a plurality of radial arms of high magnetic permeability extending from each of said members to the inner walls of said tubular member, and a plurality of electrical conductors disposed within said tubular member substantially parallel to said axis for conducting magnetizing currents, the locations of said conductors and relative orientation of said arms being such to produce periodic magnetic fields within said space having substantial components along the axis of said device.

13. In a traveling wave tube including means projecting a beam of electrons through an interaction space in energy exchanging relationshipwith waves propagating therein, means for focusing said electrons in said space and enclosing said space comprising a plurality of highly magnetically permeable annular members spaced apart along the axis of said device substantially coaxialtherewith, a plurality of annular spacers of low magnetic permability disposed between and sealed to said members, said members and spacers therebyienclosing said interaction space, a tubular member of larger crosswise dimension than said members disposed coaxial with said device enclosing said members and spacers, two radial arms of high magnetic permeability extending from each of said members to the inner walls of said tubular member, arms from adjacent members being disposed transverse to each other, and a plurality of electrical conductors disposed within said tubular member substantially parallel to said axis for conducting magnetizing currents in different directions therethrough, the locations of said conductors and directions of conduction being such to produce periodic magnetic fields within said space having substantial components along the axis of said'device.

14. In a traveling wave tube including means projecting a beam of electrons through an interaction space in energy exchanging relationship with waves propagating therein, means for focusing said electrons in said space and enclosing said space comprising a plurality of highly magnetically permeable annular members spaced apart along the axis of said device substantially coaxial therewith, a plurality of annular spacers of low magnetic permeability disposed between and sealed to said members, said members and spacers thereby enclosing said interaction space, a tubular member of larger crosswise dimension than said members disposed coaxial with said device enclosing said members and spacers, two radial arms of high magnetic permeability extending from each of said members to the inner walls of said tubular memher, arms from adjacent members being disposed transverse to each other, and four electrical conductors, disposed within said tubular member substantially parallel to said axis for conducting magnetizing currents, the 10- cations of said conductors and directions of conduction being such to produce periodic magnetic fields within said 

1. A MAGNETIC FOCUSING DEVICE COMPRISING A RELATIVELY LARGE TUBULAR HIGHLY MAGNETICALLY PERMEABLE BODY, A PLURALITY OF RELATIVELY SMALL TUBULAR HIGHLY MAGNETICALLY PERMEABLE BODIES DISPOSED COAXIALLY WITHINSAID LARGE BODY, INDIVIDUAL MAGNETICALLY PERMEABLE RADIAL SUPPORT MEMBERS COUPLING EACH OF SAID SMALL BODIES TO THE INNER WALLS OF SAID LARGE BODY,SO THAT ADJACENT SUPPORT MEMBERS ARE DISPOSED TRANSVERSE TO EACH OTHER AND MEANS CONDUCTING ELECTRIC CURRENT LONGITUDINALLY WITHIN SAID FIRST TUBULAR BODY TO INDUCE MAGNETIC FIELDS BETWEEN ADJACENT SMALL BODIES WHICH MAGNETIZE SAID TUBULAR BODY AS A QUADRUPOLAR MAGNET. 